While organic electroluminescent devices have been known for about two decades, their performance limitations have represented a barrier to many desirable applications.
Gurnee et al U.S. Pat. No. 3,172,862, issued Mar. 9, 1965, filed Sept. 29, 1960, disclosed an organic electroluminescent device. (For brevity EL, the common acronym for electroluminescent, is sometimes substituted.) The EL device was formed of an emitting layer positioned in conductive contact with a transparent electrode and a metal electrode. The emitting layer was formed of a conjugated organic host material, a conjugated organic activating agent having condensed benzene rings, and a finely divided conductive material. Naphthalene, anthracene, phenanthrene, pyrene, benzopyrene, chrysene, picene, carbazole, fluorene, biphenyl, terphenyls, quaterphenyls, triphenylene oxide, dihalobiphenyl, trans-stilbene, and 1,4-diphenylbutadiene were offered as examples of organic host materials. Anthracene, tetracene, and pentacene were named as examples of activating agents, with anthracene being disclosed to impart a green hue and pentacene to impart a red hue. Chrome and brass were disclosed as examples of the metal electrode while the transparent electrode was disclosed to be a conductive glass. The emitting layer was disclosed to be "as thin as possible, about 0.0001 inch"--i.e., 2.54 micrometers. Electroluminescence was reported at 800 volts and 2000 hertz.
Recognizing the disadvantage of employing high voltages and frequencies Gurnee U.S. Pat. No. 3,173,050 reported electroluminescence at 110 volts d.c. by employing in series with the emitting layer an impedance layer capable of accounting for 5 to 50 percent of the voltage drop across the electrodes.
Until relatively recently the art has reported at best modest performance improvements over Gurnee while resorting to increasingly challenging device constructions, such as those requiring alkali metal cathodes, inert atmospheres, relatively thick monocrystalline anthracene phosphor elements, and/or specialized device geometries. Mehl U.S. Pat. No. 3,382,394, Mehl et al U.S. Pat. No. 3,530,325, Roth U.S. Pat. No. 3,359,445, Williams et al U.S. Pat. No. 3,621,321, Williams U.S. Pat. No. 3,772,556, Kawabe et al "Electroluminescence of Green Light Region in Doped Anthracene", Japan Journal of Applied Physics, Vol. 10, pp. 527-528, 1971, and Partridge U.S. Pat. No. 3,995,299 are representative.
In 1969 Dresner, "Double Injection Electroluminescence in Anthracene", RCA Review, Vol. 30, pp. 332-334, independently corroborated the performance levels of then state of the art EL devices employing thick anthracene phosphor elements, alkali metal cathodes, and inert atmospheres to protect the alkali metal from spontaneous oxidation. These EL devices were more than 30 .mu.m in thickness and required operating potentials of more than 300 volts. In attempting to reduce phosphor layer thickness and thereby achieve operation with potential levels below 50 volts Dresner attempted to coat anthracene powder between a conductive glass anode and a gold, platinum or tellurium grid cathode, but phosphor layer thicknesses of less than 10 .mu.m could not be successfully achieved because of pinholes.
Dresner U.S. Pat. No. 3,710,167 reported a more promising EL device employing like Gurnee et al and Gurnee a conjugated organic compound, but as the sole component of an emitting layer of less than 10 .mu.m (preferably 1 to 5 .mu.m) in thickness. A tunnel injection cathode consisting of aluminum or degenerate N.sup.+ silicon with a layer of the corresponding aluminum or silicon oxide of less than 10 Angstroms in thickness was employed.
The most recent discoveries in the art of organic EL device construction have resulted from EL device constructions with two extremely thin layers (&lt;1.0 .mu.m in combined thickness) separating the anode and cathode, one specifically chosen to transport holes and the other specifically chosen to transport electrons and acting as the organic luminescent zone of the device. This has allowed applied voltages to be reduced for the first time into ranges approaching compatibility with integrated circuit drivers, such as field effect transistors. At the same time light outputs at these low driving voltages have been sufficient to permit observation under common ambient lighting conditions.
For example, Tang U.S. Pat. No. 4,356,429 discloses in Example 1 an EL device formed of a conductive glass transparent anode, a 1000 Angstroms hole transporting layer of copper phthalocyanine, a 1000 Angstroms electron transporting layer of tetraphenylbutadiene in poly(styrene) also acting as the luminescent zone of the device, and a silver cathode. The EL device emitted blue light when biased at 20 volts at an average current density in the range of 30 to 40 mA/cm.sup.2. The brightness of the device was 5 cd/m.sup.2. Tang teaches useful cathodes to be those formed from common metals with a low work function, such as indium, silver, tin, and aluminum.
A further improvement in organic layer EL devices is taught by Van Slyke et al U.S. Pat. No. 4,539,507. Referring to Example 1, onto a transparent conductive glass anode were vacuum vapor deposited successive 750 Angstrom hole transporting 1,1-bis(4-di-p-tolylaminophenyl)cyclohexane and electron transporting 4,4'-bis(5,7-di-t-pentyl-2-benzoxzolyl)stilbene layers, the latter also providing the luminescent zone of the device. Indium was employed as the cathode. The EL device emitted blue-green light (520 nm peak). The maximum brightness achieved 340 cd/m.sup.2 at a current density of about 140 mA/cm.sup.2 when the applied voltage was 22 volts. The maximum power conservation efficiency was about 1.4.times.10.sup.-3 watt/watt, and the maximum EL quantum efficiency was about 1.2.times.10.sup.-2 photon/electron when driven at 20 volts. Silver, tin, lead, magnesium, manganese, and aluminum are specifically mentioned for cathode construction.
Van Slyke et al U.S. Ser. No. 013,528, filed Feb. 11, 1987, commonly assigned, titled ELECTROLUMINESCENT DEVICE WITH ORGANIC LUMINESCENT MEDIUM, discloses an organic EL device comprised of, in the sequence recited, an anode, an organic hole injecting and transporting zone, an organic electron injecting and transporting zone, and a cathode. The organic EL device is further characterized in that the organic hole injecting and transporting zone is comprised of a layer in contact with the anode containing a hole injecting porphyrinic compound and a layer containing a hole transporting aromatic tertiary amine interposed between the hole injecting layer and the electron injecting and transporting zone.
Tang et al U.S. Ser. No. 013,530, filed Feb. 11, 1987, commonly assigned, titled ELECTROLUMINESCENT DEVICE WITH IMPROVED CATHODE, discloses an EL device comprised of a cathode formed of a plurality of metals other than alkali metals, at least one of which has a work function of less than 4 eV.